Near-field scanning optical microscopy (“NSOM”) is a key technology for optically imaging nanoscale features in the sub-200 nanometer range. Conventional optical microscopy is limited in resolution to about one-half a wavelength of the illuminating light, NSOM microscopes, in contrast, can resolve features much smaller than the illuminating wavelength. For example, it is very difficult to obtain sub-500 nanometer resolution with conventional microscopes, but NSOM microscopes can resolve features of 25 nanometers or less. Such high resolution permits images of important macromolecules and cell components.
When an object is illuminated with electromagnetic radiation in the form of light it interacts with the radiation and responds with radiation components of two types: traveling wave radiation which travels many wavelengths distance with relatively small attenuation (far-field radiation) and evanescent radiation which attenuates exponentially with distance from the object and typically drops to undetectable levels within a distance of a few wavelengths from the object (near-field radiation). In imaging an object, conventional optical microscopy uses only far-field radiation that has traveled several wavelengths or more from the object. Consequently the image is created by only the traveling waves. Without the further information provided by the near-field radiation, nanoscale features are not resolved. These details are obscured by diffraction of the far-field waves.
NSOM microscopes, in contrast, position a probe within a fraction of wavelength from the object. This close to the object, the near-field light has measureable effects. The probe interacts with the object to produce measureable effects on the far-field response. These measureable effects can be seen by spectral analysis of the light returned from the illuminated sample/probe system. By scanning the probe laterally over the sample, the effects can be measured for each point over the sample and the resulting measurements can be processed into an image by appropriate “deconvolution algorithms” to provide an image of nanoscale features.
Referring to the drawings, FIG. 9 is a schematic diagram of a typical NSOM microscope comprising an illumination light source 10, a sample stage 11, and a near-field optical probe 12. Positioning mechanism 13 positions the probe 12 a fraction of the illumination wavelength from an object 14 disposed on the sample stage 11, and a scanner 15 scans the probe 12 over a two-dimensional area of the sample.
The light source 10 is typically a laser of substantially monochromatic wavelength λ focused into an optical fiber 19 through a polarizer, a beam splitter and a coupler (collectively not shown). The polarizer and the beam splitter serve to remove stray light from returning reflected light.
The sample stage 11 typically comprises a planar surface supporting the sample 14. The stage 11 can be fixed where the probe is scanned over the sample in two lateral dimensions (“x and y”) or the stage 11 can be a moveable portion of a scanning mechanism that moves the sample in one or two of the lateral dimensions.
Conventionally the near-field optical probe is an optical fiber with a very sharp, small diameter point 12. It may be coated with metal except at the point. The portion of the probe adjacent the sample (the active region of the probe) typically has an effective lateral diameter smaller than the wavelength.
The positioner 13 positions the probe tip 12 a fraction of a wavelength λ over the sample object 14. The positioner is typically a feedback mechanism. One type of positioner uses a mechanical beam-deflection set-up (not shown) to provide a cantilevered probe. The normal force is monitored using the beam deflection set-up.
The scanner 15 scans the probe in one lateral dimension (e.g. the “y” dimension) and the stage itself can be moved in a second lateral dimension (e.g. the “x” dimension).
The optical detector 16 detects and analyzes light from the illuminated probe/sample. The detector can be a standard optical detector such as an avalanche photodiode, a photomultiplier tube or a charge-coupled device (CCD). The output of the detector 16 is typically processed by a computer 17 programmed with a deconvolution program, and the results are presented on a display 18 as an image.
Depending on the sample being imaged, there are multiple possible modes of operation for a NSOM. In transmission mode operation (“TM”), which is illustrated in FIG. 9, light from the source 10 travels through the probe and transmits through the sample. TM requires a transparent sample. In reflection mode 0peration (“RM”), light travels through the probe and reflects from a sample surface. RM allows for opaque samples. In collection mode operation (“CM”), the sample is illuminated from an outside light source and the probe collects the reflected light. In CM, the probe both illuminates the sample and collects the reflected light. The present invention has application to respective microscopes using each of these modes of operation.
The microscope can be set up to image from any one of several contrast mechanisms including polarization, topography, birefringence, index of refraction, fluorescence, wavelength dependence and reflectivity. Further details concerning the structure and operation of various NSOM systems can be found in the publication L. Novotny and B. Hecht, Principles of Nano-optics (Cambridge University Press, Cambridge, 2006), which is incorporated herein by reference.
The optical probe is the key component determining the resolution of the NSOM. When illuminated, it provides a light source with an effective diameter that is small compared with the wavelength of the illumination light and is positioned by the feedback mechanism much closer to the sample surface than the wavelength of the illumination light.
FIG. 10 illustrates a conventional pulled fiber probe 20 providing an illuminated tip 21 closely adjacent the surface of a sample 22. The illuminated tip 21 can optionally contain a nanoparticle 23. The effective diameter of the conventional probe apex (with or without the nanoparticle) is smaller than a wavelength of the illuminating light (it is typically 5-100 nm). The tip 21 is disposed by the feedback mechanism (not shown) to less than an illumination wavelength from the surface of the sample 22 (typically 5-100 nm). FIG. 10 (a) illustrates an apertureless NSOM probe. For better confinement of light, the tip (20) can be coated with a metal film 24 as shown in FIG. 10(b).
Conventional thinking was that obtaining super resolution with NSOM microscopes requires super miniaturization of the probe. It was commonly accepted that in order to increase spatial resolution, the light electromagnetic field should be maximally localized. This was achieved by using extremely small probes in the form of sharp glass or metal tips, fluorescent nanoparticles and dielectric and metal corners. Such miniaturized tips, however, have presented serious problems. First, they are fragile. Fragile tips break and even become unwanted artifacts on the sample. Moreover, in illumination, maximum localization of power on such small areas creates substantial local heating which can change critical dimensions and optical properties. In collection, the miniature probes can collect only a small portion of the light scattered from the sample. Accordingly there is a need for a NSOM having a more robust probe that can collect a large portion of the scattered light while permitting nanoscale resolution.